A number of various types of data communication networks are utilized for communicatively coupling together data processing systems. Examples of such data communication networks are RS-232, RS-422 and Ethernet. In general, such communication networks are characterized by a synchronous or an asynchronous bit serial transmission of data along a signal line of a communications bus. Each node, such as a data processing system, coupled to the communication bus typically has data transceivers for transmitting data to the bus and for receiving data therefrom.
In addition, a number of different network topologies are available such as a ring topology or a star topology. In some data communication networks, there is a central controlling data element which synchronizes and controls the transmission of data between the various nodes interconnected to the communication bus. Other types of networks employ a distributed type of structure wherein each node operates asynchronously to every other node.
As can be appeciated, in any such data communications network it is essential that two nodes do not attempt to transmit data simultaneously. Such an occurrence is referred to as a collision and results in the destruction of the data and, typically, in the requirement that each of the transmitting nodes once more begin their transmission. It can be further appreciated that a collision tends to reduce the effective bandwidth of the bus.
A variety of different techniques for avoiding collisions or reducing the probability of a collision have been devised. Some of the techniques involve sampling the data bus itself to determine whether or not data is being transmitted at any given time across the bus. It has also been known to provide, in conjunction with the sampling of the data bus, a dedicated signal line for indicating a state of the bus.
In any such collision avoidance technique it is important that the cost and complexity of the technique do not overly burden the system. It is also important that the technique be readily implemented and readily allow for the expansion of the bus such as when additonal nodes are added to the communications bus after the initial installation of the bus.